Low back pain is a common disease for example caused by herniated discs, compressed nerve roots, degenerative discs or joint disease.
If a patient has this severe pain and does not respond to conservative treatment, spinal fusion is an option to eliminate the pain. Spinal fusion is a surgical technique, wherein two or more vertebrae are joined together. Spinal fusion interventions are also performed to correct back deformities.
With inter-body fusion often an intervertebral spacer or device is placed between the involved vertebrae after removal of the intervertebral disc. The intervertebral device corrects the spine alignment and restores the disc height.
Common intervertebral devices are made from titanium alloys or polyetheretherketone (PEEK). Often these devices comprise pockets that can be filled with bone graft material or artificial graft substitute. The fusion itself takes place when the bone of the endplates grows into and through the intervertebral device. Finally both vertebrae are grown together. Often, additionally, a pedicle system provides additional posterior stabilisation. Intervertebral fusion devices can be implanted with various approaches, for example from the anterior, the posterior or the lateral side.
Over the past years minimal invasive techniques have been introduced. The advantages of the minimal invasive techniques are less soft tissue trauma resulting in a faster recovery. Other complications are reduced as well. In minimal invasive techniques the implant is brought into position between the vertebrae through a small incision with small instruments. Still the intervertebral device must have a sufficient large foot-print to translate the forces between the vertebrae before complete fusion has taken place. If a device is too small, it will sink into or break through the endplate of the vertebra, and the initially restored height is lost.
Combining advantages of the minimal invasive surgery approaches with well supporting intervertebral devices with large foot-print, a device would be required that can be brought into place through a small incision, and in a second step is expanded to a larger size.
Various embodiments of such devices are known in the art. For example, WO 2009/098536 (Calvosa Giuseppe) discloses an intervertebral distractor comprising stop members and which may be percutaneously implanted. The distractor comprises an elongated body adapted to provide an interspinous support between two adjacent spinous processes as well as a first and a second couple of mobile stabilizers which are adapted to rotate from a closed position to a spread apart position. Further, the distractor includes means for bringing said stabilizers from the closed position into the spread apart position, wherein said means are movable along a longitudinal axis of said distractor and being coupled to one end of each of said stabilizers to rotate said stabilizers from said closed position to said spread apart position.
U.S. 2009/0048676 A1 (Henry Fabian JR) describes an implant comprising two members being pivotal relative to each other and each comprising two limbs. The implant may be inserted into a vertebral space in a first non-expanded configuration, where all limbs are adjacent to each other, and which may then subsequently be deployed to a second expanded configuration having a larger foot-print, where the limbs are separated from each other. The implant may further comprise a locking mechanism used to lock the position of the first member relative to the second member. Deployment of the implant from the first configuration to the second configuration is performed by a cable which may be pulled by a surgeon such as to expand the implant.
U.S. 2012/0029639 discloses interbody spinal implants being implantable into a patient in a first orientation having a reduced height and then rotated to a second orientation which extends the height of the implant.
The implants known in the art have the disadvantage that the expansion of the different implant parts may not be equal due to different forces acting on each part. For example, the use of a cable exerts a maximal force onto one part which is directly coupled to the cable, while the force exerted on other parts may be diminished by external forces acting on these parts.